In recent years, in view of environmental issues, high oil price, etc., there is an increasing demand for energy-saving-oriented industrial products. This also applies to the field of construction vehicles, work vehicles, etc., which have conventionally been mainly based on a hydraulic drive system using a diesel engine. There are many instances in which high efficiency and energy saving have been achieved through electrification. For example, in the case where the drive portion of a construction vehicle or a work vehicle is electrified, that is, in the case where an electric motor is employed as the drive source, there are to be expected, apart from a reduction in exhaust gas, many energy-saving effects, such as a high-efficiency drive of the engine (in the case of a hybrid type machine), an improvement in terms of power transmission efficiency, and recovery of regenerated electric power. Thus, in the field of the hydraulic excavator, engine-driven forklift, etc., there are being commercialized “hybrid vehicles” in which a diesel engine and an electric motor are combined with each other.
Among such construction vehicles and work vehicles, the drive portions of which have been electrified, is a wheel loader, of which a relatively high fuel reducing effect is to be expected when it is produced as a hybrid vehicle. The conventional wheel loader is, for example, a work vehicle which excavates and transports earth and sand or the like with a bucket portion of a hydraulic work unit mounted to the front portion of the vehicle while performing traveling by transmitting the power of an engine to tires via a torque converter and a transmission (T/M). If the traveling drive portion of such a wheel loader is electrified, it is possible to improve the power transmission efficiency of the traveling unit consisting of a torque converter, a transmission, and the like to the level of power transmission efficiency based on electricity. Further, in the wheel loader, the traveling operation of starting/stopping is frequently repeated, so that, if the traveling drive portion is electrified, recovery of regenerated electric power from the traveling electric motor at the time of braking is to be expected.
Regarding such a hybrid wheel loader, there has been disclosed one in which the power supplied from the engine and the power storage device is properly controlled in order to suppress the deterioration in driving comfort attributable to the distribution of the power to the hydraulic pump and the traveling electric motor (see, for example, Patent Document 1). More specifically, when the demanded power of the work vehicle is larger than the available output power of the hybrid system, the actual hydraulic power of the hydraulic pump is increased to the required value under a certain limitation and, at the same time, the traveling power is reduced from the required value by a value equal to or less than the limitation.